Various terms are defined in the following specification. For convenience, a Glossary of terms is provided herein, immediately preceding the claims.
Frequently, there is a need to store and transport pressurized, volatile fluids at cryogenic temperatures, i.e., at temperatures lower than about -40.degree. C. (-40.degree. F.). For example, there is a need for containers for storing and transporting pressurized liquefied natural gas (PLNG) at pressures in the broad range of about 1035 kPa (150 psia) to about 7590 kPa (1100 psia) and at temperatures higher than about -123.degree. C. (-190.degree. F.). There is also a need for containers for safely and economically storing and transporting other pressurized fluids, such as methane, ethane, and propane, at cryogenic temperatures. For such containers to be constructed of a welded steel, the steel and its weldments (see Glossary) must have adequate strength to withstand the fluid pressure and adequate toughness to prevent initiation of a fracture, i.e., a failure event, at the operating conditions.
As will be familiar to those skilled in the art, the Charpy V-notch (CVN) test can be used for the purpose of fracture toughness assessment and fracture control in the design of storage containers for transporting pressurized, cryogenic temperature fluids, such as PLNG, particularly through use of the ductile-to-brittle transition temperature (DBTT). The DBTT delineates two fracture regimes in structural steels. At temperatures below the DBTT, failure in the Charpy V-notch test tends to occur by low energy cleavage (brittle) fracture, while at temperatures above the DBTT, failure tends to occur by high energy ductile fracture. Storage and transportation containers that are constructed from welded steels for the aforementioned cryogenic temperature applications and for other load-bearing, cryogenic temperature service must have DBTTs, as determined by the Charpy V-notch test, well below the service temperature of the structure in order to avoid brittle failure. Depending on the design, the service conditions, and/or the requirements of the applicable classification society, the required DBTT temperature shift (i.e., how far the DBTT must be below the intended service temperature) may be from 5.degree. C. to 30.degree. C. (9.degree. F. to 54.degree. F.) below the service temperature.
Nickel-containing steels conventionally used for cryogenic temperature structural applications, e.g., steels with nickel contents of greater than about 3 wt %, have low DBTTs, but also have relatively low tensile strengths. Typically, commercially available 3.5 wt % Ni, 5.5 wt % Ni, and 9 wt % Ni steels have DBTTs of about -100.degree. C. (-150.degree. F.), -155.degree. C. (-250.degree. F.), and -175.degree. C. (-280.degree. F.), respectively, and tensile strengths of up to about 485 MPa (70 ksi), 620 MPa (90 ksi), and 830 MPa (120 ksi), respectively. In order to achieve these combinations of strength and toughness, these steels generally undergo costly processing, e.g., double annealing treatment. In the case of cryogenic temperature applications, industry currently uses these commercial nickel-containing steels because of their good toughness at low temperatures, but must design around their relatively low tensile strengths. The designs generally require excessive steel thicknesses for load-bearing, cryogenic temperature applications. Thus, use of these nickel-containing steels in load-bearing, cryogenic temperature applications tends to be expensive due to the high cost of the steel combined with the steel thicknesses required.
Current commercial storage containers for transportation of liquefied natural gas at -162.degree. C. (-260.degree. F.) and atmospheric pressure (LNG) are typically constructed of the above-mentioned commercial nickel-containing steels, austenitic stainless steels, or aluminum. In LNG applications, the strength and toughness requirements for such materials, and for weldments joining such materials, are distinctly different from those for the PLNG case. For example, in discussing the welding of 21/4 wt % to 9 wt % Ni steels for cryogenic purposes, G. E. Linnert, in "Welding Metallurgy", American Welding Society, 3rd Ed., Vol. 2, 1967, pp. 550-570, lists the Charpy V-notch toughness (see Glossary) requirements for such weldments as ranging from about 20 J to 61 J as measured at the service temperature. Also, the 1995 publication, Det Norske Veritas (DNV) Rules For Classification of Ships, specifies that materials used in new-built, liquefied gas carrying ships must meet certain minimum Charpy V-notch toughness requirements. Specifically, the DNV Rules state that plates and weldments used for pressure vessels with design temperatures ranging from -60.degree. C. to -165.degree. C. must meet a minimum Charpy toughness of 27 J at test temperatures ranging from 5.degree. C. to 30.degree. C. (9.degree. F. to 54.degree. F.) below the design temperature. The requirements listed by Linnert and the DNV Rules cannot be directly applied to the construction of containers for transportation of PLNG (or other pressurized, cryogenic fluids) since the PLNG containment pressure, typically about 2760 kPa (400 psia), is significantly higher than for conventional methods of transporting LNG, which is generally at or near atmospheric pressure. For PLNG storage and transportation containers, there is a need for more stringent toughness requirements, and therefore, a need for weldments with better toughness properties than those now used for constructing LNG storage containers.
Base Plate Material
Storage containers for pressurized, cryogenic temperature fluids, such as PLNG, are preferably constructed from discrete plates of an ultra-high strength, low alloy steel. Three co-pending U.S. patent applications identify various weldable, ultra-high strength, low alloy steels with excellent cryogenic temperature toughness for use in constructing storage containers for transporting PLNG and other pressurized, cryogenic temperature fluids. The steels are described in a co-pending U.S. patent application entitled "ULTRA-HIGH STRENGTH STEELS WITH EXCELLENT CRYOGENIC TEMPERATURE TOUGHNESS", which has a priority date of Dec. 19, 1997 and is identified by the United States Patent and Trademark Office ("USPTO") as application Ser. No. 09/099,649 and has been published in WO 99/32672 in a co-pending U.S. patent application entitled "ULTRA-HIGH STRENGTH AUSAGED STEELS WITH EXCELLENT CRYOGENIC TEMPERATURE TOUGHNESS", which has a priority date of Dec. 19, 1997 and is identified by the USPTO as application Ser. No. 09/099,153 and has been published in WO 99/32670 and in a co-pending U.S. provisional patent application entitled "ULTRA-HIGH STRENGTH DUAL PHASE STEELS WITH EXCELLENT CRYOGENIC TEMPERATURE TOUGHNESS", which has a priority date of Dec. 19, 1997 and is identified by the USPTO as application Ser. No. 09/099,152 and has been published in WO 99/32671. These steels are especially suitable for many cryogenic temperature applications, including transportation of PLNG, in that the steels have the following characteristics for steel plate thicknesses of preferably about 2.5 cm (1 inch) and greater: (i) DBTT lower than about -73.degree. C. (-100.degree. F.), preferably lower than about -107.degree. C. (-160.degree. F.), in the base steel and in the weld HAZ, (ii) tensile strength greater than 830 MPa (120 ksi), preferably greater than about 860 MPa (125 ksi), and more preferably greater than about 900 MPa (130 ksi), (iii) superior weldability, (iv) substantially uniform through-thickness microstructure and properties, and (v) improved toughness over standard, commercially available, ultra-high strength, low alloy steels. The steels described in the above-mentioned co-pending U.S. provisional patent applications may have a tensile strength of greater than about 930 MPa (135 ksi), or greater than about 965 MPa (140 ksi), or greater than about 1000 MPa (145 ksi). Other suitable steels are described in a European Patent Application published Feb. 5, 1997, and having International application number: PCT/JP96/00157, and International publication number WO 96/23909 (08.08.1996 Gazette 1996/36) (such steels preferably having a copper content of 0.1 wt % to 1.2 wt %), and in a co-pending U.S. patent application entitled "ULTRA-HIGH STRENGTH, WELDABLE STEELS WITH EXCELLENT ULTRA-LOW TEMPERATURE TOUGHNESS", which has a priority date of Jul. 28, 1997 and is identified by the USPTO as Application Ser. No. 09/123,625 and has been published in WO 99/05335.
Welding
Such steels may be joined together to form storage containers for pressurized, cryogenic temperature fluids, such as PLNG, by a welding method suitable for producing a weldment that provides adequate strength and fracture toughness for the intended application. Such a welding method preferably includes a suitable welding process, for example without limitation, gas metal arc welding ("GMAW"), tungsten inert gas ("TIG") welding, or submerged arc welding ("SAW"); a suitable welding consumable wire; a suitable welding consumable gas (if required); a suitable welding flux (if required); and suitable welding procedures, for example without limitation, preheat temperatures, and welding heat inputs. A weldment is a welded joint, including: (i) the weld metal, (ii) the heat-affected zone ("HAZ"), and (iii) the base metal in the "near vicinity" of the HAZ. The weld metal is the welding consumable wire (and flux, if used) as deposited and diluted by the portion of the base metal plate that melts during performance of the welding process. The HAZ is the portion of the base metal that does not melt during welding, but whose microstructure and mechanical properties are altered by the heat of the welding process. The portion of the base metal that is considered within the "near vicinity" of the HAZ, and therefore, a part of the weldment, varies depending on factors known to those skilled in the art, for example without limitation, the width of the weldment, the dimensions of the base metal plate that is welded, and the distances between weldments.
Properties of Weldments Desired for PLNG Applications
For the purpose of constructing storage containers for PLNG and other pressurized, cryogenic temperature fluids, it is desirable to have a welding method, including a welding consumable wire, a welding consumable gas, a welding process, and welding procedures that will provide weldments with tensile strengths and fracture toughnesses suitable for the intended cryogenic application, according to known principles of fracture mechanics, as described herein. More particularly, for constructing storage containers for PLNG, it is desirable to have a welding method that will provide weldments with tensile strengths greater than about 900 MPa (130 ksi) and fracture toughnesses suitable for the PLNG application according to known principles of fracture mechanics, as described herein. The tensile strength of such weldments is preferably greater than about 930 MPa (135 ksi), more preferably greater than about 965 MPa (140 ksi), and even more preferably at least about 1000 MPa (145 ksi). Current commercially available welding methods using commercially available welding consumable wires are not suitable for welding the aforementioned high strength, low alloy steels and providing weldments with the desired properties for commercial cryogenic, pressurized applications.
Consequently, the primary objects of the present invention are to improve the state-of-the-art welding technology for applicability to ultra-high strength, low alloy steels so as to provide a welding method that will produce weldments that have tensile strengths greater than about 900 MPa (130 ksi) and fracture toughnesses suitable for the intended cryogenic application according to known principles of fracture mechanics, as described herein.